1. Field of the Invention
The invention relates to a porous, liquid-permeable composite article useful as a diaphragm for electrolysis or as a filtering medium.
2. Description of Related Art
In the electrolysis or electrosynthesis of chemical compounds, a porous diaphragm is often used to separate the anode and cathode compartments and the reaction products while permitting the flow of some liquid components from one compartment to another. For example, in the production of chlorine and sodium hydroxide from brine, the brine feed flows from the anode compartment through the porous diaphragm to the cathode compartment and then is discharged from the cell as illustrated in FIG. 1. Approximately half of the sodium chloride is converted to sodium hydroxide and chlorine in the process.
The effect of diaphragm structure on the performance of a chlor-alkali cell is quite complex. The diaphragm can be described in terms of pore size distribution, porosity, tortuosity, thickness and resultant permeability of the structure. For a given set of cell operating conditions, these parameters, and especially their uniformity across the active area of the diaphragm, determine the electrical energy usage of the cell. Lack of uniformity of flow rates across the surface of the diaphragm cause areas of low brine velocity, which allows hydroxyl ion back migration, leading to poor current efficiency. This effect can be ameliorated by using a thicker, less porous or more tortuous structure, leading however, to higher operating voltage and greater electrical energy usage. The art in designing a diaphragm for chlor-alkali production is to properly balance the diaphragm properties to minimize overall electrical energy usage by reducing operating voltage while maintaining high current efficiency. This is most effectively done with a diaphragm whose properties are highly uniform across its active area.
In the electrolysis of brine, the power consumption in terms of kilowatt hour (kWh) per metric ton of sodium hydroxide can be expressed by the following equation: ##EQU1##
Obviously it is economically desirable to achieve as high a caustic current efficiency as possible and require as low a voltage as possible. A desirable diaphragm will have a low "k" factor, k being the slope of the voltage versus current density relationship. Many commercial diaphragm chlor-alkali cells operate at a maximum current density of either 2.3 kilo-amperes per square meter or 2.8 kilo-amperes per square meter.
A commonly used porous diaphragm is prepared from asbestos fiber by essentially a paper making process. Asbestos flock is slurried and deposited in place on a screen in the electrolyzer to form a relatively thick, stiff diaphragm which is held together by the hydroxide gels formed in the asbestos while in operation. The asbestos diaphragm has often been termed a "living diaphragm" in that it is constantly being changed by dissolution, erosion, redeposition and precipitation of silica and alkali earth hydroxides. Suspended particles and dissolved alkali earth metals redeposit preferentially in the higher flow regions allowing a leveling effect on permeability.
However, this "living" or reactive feature of the asbestos diaphragm can contribute to a relatively short life. Usually, within 6-12 months, sufficient chemicals are leached out of the asbestos and the uniformity and porosity are so degraded that current efficiency drops to unacceptable levels. For the same reason, electrical upsets or fluctuations in the system can result in a very rapid destruction of the asbestos diaphragm. Finally, the asbestos diaphragm has a relatively high k factor of about 0.55 volt square meter per kiloampere (Vm.sup.2 /kA) such that, for example, at 2.8 kA/m.sup.2, the diaphragm typically requires 3.84 volts or more for operation, resulting in substantial power costs. For these reasons, the industry has sought more inert, more stable diaphragms, which can operate consistently at high current efficiency and at lower voltage and which will not be destroyed by power upsets, fluctuations or outages.
A number of modified asbestos diaphragms have been developed. U.S. Pat. No. 3,853,720 discloses a preparation of a diaphragm for chlor-alkali service involving asbestos fiber, a second fibrous material including polytetrafluoroethylene (PTFE) fiber, and an organic exchange resin. U.S. Pat. Nos. 4,170,537, 4,170,538 and U.S. Pat. No. 4,170,539 describe a diaphragm of an asbestos or polymer matrix containing inorganic zirconium or magnesium compounds and, in some cases, "Nafion.RTM. 601 polymer solution", a colloidal dispersion of hydrolyzed perfluorosulfonic acid polymer, which is used to impregnate the structure. All of these structures rely, to some extent, on the asbestos or the added compounds which generate hydroxide gels to regulate porosity and uniformity. Accordingly, though somewhat more stable than unmodified asbestos, they suffer from the same deficiencies described for the asbestos diaphragm described above. These patents also mention expanded PTFE (EPTFE) as a possible polymer matrix but the pore size range specified, 0.8-50 microns, preferably 5-20 microns, is relatively large so that the permeability level is controlled by the hydroxide gels formed within the diaphragm.
A number of U.S. Patents, U.S. Pat. Nos. 3,930,979, 4,113,912, 4,224,130, 4,606,805, 4,385,150, 4,666,573, 4,680,101, 4,720,334 and 4,341,614, describe porous PTFE diaphragms made wettable by various means. U.S. Pat. Nos. 3,930,979, 4,250,002, 4,113,912, 4,385,150 and 4,341,614, describe porous PTFE diaphragms prepared by combining PTFE powder or fiber with a sacrificial filler. The mixture is formed into a sheet and the filler is removed by dissolving it or decomposing it with heat thus leaving the PTFE sheet porous. The homogeneity of the mixture and the particle size distributions of the filler and PTFE severely limit the uniformity possible in the finished diaphragm. Because a large percentage of the structure is removed to provide the necessary porosity, the finished diaphragm is inherently weak. To offset these uniformity and strength problems, the finished diaphragm must be very thick resulting in high operating voltage. A number of patents, including U.S. Pat. Nos. 4,606,805, 4,666,573, 4,113,912, 4,680,101 and U.S. Pat. No. 4,720,334, describe porous PTFE diaphragms prepared by a PTFE fiber slurry deposition process. The size, shape and size distribution of the PTFE fiber available leads to a large pore, inherently weak, non-uniform structure which must be made very thick to provide utility. This trade-off results in high operating voltages. In addition, in U.S. Pat. Nos. 3,930,979, 4,113,912, 4,224,130, 4,250,002, 4,341,614 and 4,606,805, no perfluoro ion exchange polymer is involved so that an adequate level of hydrophilicity, which remains chemically stable in a hostile environment such as a chlor-alkali cell, is not achieved. When an adequate level of hydrophilicity is not maintained, gas bubbles generated at the cathode will accumulate in the diaphragm pores blocking both bulk and ion flow. This reduces the effective diaphragm area leading to an increase in operating voltage and eventually causing system shutdown. This is called "gas locking".
U.S. Pat. No. 3,944,477 describes a diaphragm of porous polytetrafluoroethylene sheet material with a microstructure characterized by nodes and fibrils and having a multilayer structure wherein a number of such films are bonded together. Initial wettability is achieved by treatment with acetone and water, with no mention made of organic surfactants. More importantly, there is no disclosure of impregnation with a perfluoro ion exchange polymer. While the diaphragms of the present invention have given satisfactory performance after as much as 421 days, the reference reports no extended runs.
U.S. Pat. Nos. 4,089,758 and 4,713,163 describe porous diaphragms involving EPTFE structures made hydrophilic with inorganic filler particles or organic surfactants. These diaphragms are susceptible to gas locking, which will block ion and bulk fluid flow, resulting in increasing operating voltage, decreasing current efficiency and ultimate system shutdown.
U.S. Pat. No. 3,940,916 describes a porous diaphragm made from a fabric spun from an ion exchange polymer. The pore size of this structure is too large for efficient operation in a chlor-alkali cell. Because of this, very thick structures are required for high current efficiency operation resulting in high voltage.
U.S. Pat. No. 4,385,150 discloses, but does not claim, a porous asbestos or PTFE substrate impregnated with an organic solution of a fluorinated copolymer having a carboxyl functional group. The disclosure does not specify the PTFE as having a microstructure characterized by a series of nodes interconnected by fibrils, nor does it specify a multilayer construction. Accordingly, this PTFE structure does not provide the uniformity of structure, porosity and permeability necessary for sustained high efficiency operation in a chlor-alkali cell.
U.S. Pat. Nos. 3,692,569, 4,453,991, 4,865,925 and 4,348,310 and Japanese Patent Applications JPA-61-246,394 and JPA-63-99,246 describe and claim porous diaphragms involving EPTFE coated with perfluoro ion exchange resin for use in electrochemical cells. However, the EPTFE cited is not a layered structure and will not, at a corresponding thickness, provide the small pore size and uniformity of structure necessary for efficient operation of a chlor-alkali cell, especially in thicknesses exceeding 20 mils. Moreover, U.S. Pat. Nos. 4,453,991 and 4,348,310 specify impregnation of the EPTFE with perfluoro ionomer solutions with equivalent weights exceeding 1000. The relatively large micelles of these high equivalent weight systems cannot penetrate a relatively thick, small pored EPTFE structure to uniformly and thoroughly impart sufficient hydrophilicity to the entire structure to allow efficient chlor-alkali diaphragm operation.
U.S. Pat. No. 4,865,925 discloses use of an EPTFE/perfluoro ion exchange resin structure for a fuel cell, which is an electrochemical cell, but the structure would not be suitable for a diaphragm because it is porous to gas. A chlor-alkali cell requires that hydrogen from the cathode side must not mix with chlorine from the anode side because most hydrogen/chlorine mixtures are explosive. The reference makes no disclosure of the need for thorough impregnation, the need for at least four layers of EPTFE thermally bonded together, or the preference for certain equivalent weights for the ion exchange resin.
U.S. Pat. No. 4,277,429 describes a method for producing a porous PTFE that is asymmetric in the sense that there is a measurable difference in bubble point between one surface and the reverse side surface. Slightly different permeability to isopropanol is noted in one direction than in the reverse direction. Such a structure, however, is monolithic and not layered and would not provide the high level of uniformity of pore size and pore size distribution necessary for efficient operation of a chlor-alkali cell. In addition, the interior and exterior surfaces of this porous PTFE are hydrophobic and would "gas lock" in chlor-alkali production or in other electrolytic or filtration uses where gas entrainment is a potential problem.
International Patent Application No. PCT/US88/00237, publication number WO88/05687 and its counterpart U.S. Pat. No. 4,863,604, describes a microporous, asymmetric, integral, composite polyfluorocarbon membrane of two or more sheets of microporous fluorocarbon polymer having different average pore sizes. These structures, however, would not be useful as diaphragms in a chlor-alkali cell. Such sheets are not EPTFE structures but rather are prepared by incorporating a particulate, inorganic, solid, pore forming filler, removeable by leaching and heating, into the polytetrafluoroethylene polymer, and shaping the resultant mixture by preforming and calendering it into a self-sustaining sheet or film. The multilayer structure is assembled by starting with a sheet of PTFE/pore forming filler which has small pore forming filler particles. On top of this sheet are layered additional sheets of PTFE/pore forming filler which contain progressively larger pore forming filler particles. The sheets are bonded with heat and pressure, followed by sintering. Finally, the pore forming filler is removed by leaching or heat, thus leaving the multilayer PTFE sheet porous. As discussed above, the homogeneity of the mixture and the particle size distributions of the filler and PTFE severely limit the uniformity possible in microporous structures prepared by leaching or otherwise removing incorporated particles. This limitation is further compounded by the limitation on sharply fractionating particle sizes for the various layers of the asymmetric structure. Also, as mentioned above, because a large percentage of the structure is removed to provide the necessary porosity, the finished structure is inherently weak. To deal with these uniformity and strength problems, the finished diaphragm must be very thick, which is undesirable in electrolytic operations because of the high operating voltage required. In addition, in PCT/US88/00237 (WO88/05687) and in U.S. Pat. No. 4,863,604, no perfluoro ion exchange polymer is involved so that an adequate level of hydrophilicity which remains stable in a hostile environment is not achieved. As discussed above, when an adequate level of hydrophilicity is not maintained, "gas locking" occurs reducing effective diaphragm area leading to an increase in operating voltage.
U.S. Pat. No. 4,385,093 describes a porous PTFE article prepared by layering together PTFE components followed by expanding in one or more directions. The resulting article has very high interlayer bond strengths and appears uninterrupted at the layer interfaces. The interlayer bond strength of this article was shown to be much higher than a composite prepared by layering and sintering two already expanded PTFE sheets. The high interlayer bond strength is desirable in certain applications to prevent delamination of the layers due to gas, liquid or osmotic pressure which may build up inside the structure during use. The method of U.S. Pat. No. 4,385,093 does not take advantage of the averaging effect of layering because the expansion is carried out after the layering step. The resulting product is less uniform in structure and pore size than the article of this invention which is made by layering already expanded sheets. Further, the pure PTFE structure of U.S. Pat. No. 4,385,093 has an inherent tendency to entrain gas in certain aqueous, electrolytic or filtration applications and will eventually gas lock.
U.S. Pat. No. 4,341,615 and U.S. Pat. No. 4,410,638 both claim a wettable, microporous diaphragm for electrolysis having a base of fluorinated resin, the pores of the microporous diaphragm having deposited therein a copolymer of an unsaturated carboxylic acid and a non-ionic unsaturated monomer. The structure is monolithic and not layered and does not provide the uniformity of structure necessary to provide high current efficiency and low operating voltage for chlor-alkali operation. This deficiency is further compounded in that this monolithic, fluorinated resin construction is prepared by leaching out calcium carbonate particulates from the fluorinated resin composite. Accordingly, as pointed out earlier, the homogeneity of the mixture and particle size distribution of the filler and resin severely limit the uniformity of structure possible in the microporous sheet. The copolymer deposited within the pores to impart hydrophilicity is not perfluorinated and does not provide the necessary durability in the corrosive environment of chlor-alkali service.
The present invention is a porous, multilayer construction comprising multiple layers of porous EPTFE bonded together wherein the internal and external surfaces are at least partially coated with a perfluoro ion exchange polymer. Two U.S. patent applications, U.S. Ser. No. 206,884 and U.S. Ser. No. 278,224 now U.S. Pat. No. 4,902,308 and U.S. Pat. No. 4,954,388 in the names of some of the inventors of the present invention, involve the same starting materials.
U.S. Ser. No. 206,884, "Composite Membrane" discloses a thin porous expanded PTFE whose internal and external surfaces are coated with a metal salt of a perfluoro ion exchange polymer. That composite is porous like the present invention but, in contrast to the present invention, it is much thinner. In that application, the perfluoro ion exchange polymer serves as an anchor for active metal ions which may scavenge, catalyze or otherwise react with fluids passing through the porous structure. The EPTFE component is a single layer construction, not the multilayer form of the present invention.
U.S. Ser. No. 278,224 is a multilayer composite comprising a reinforcing fabric bonded to an expanded PTFE film which is laminated to a continuous film of perfluoro ion exchange polymer. In contrast to the present invention, that construction is a non-porous composite where the EPTFE is used as an interlayer between the continuous film of perfluoro ion exchange polymer and a reinforcing fabric. In addition, the EPTFE component is a single layer, not the multilayer construction of the present invention.
The carboxylic polymers with which the present invention is concerned have a fluorocarbon backbone chain to which are attached the functional groups or pendant side chains which in turn carry the functional groups. When the polymer is in melt-fabricable form, the pendant side chains can contain, for example, ##STR1## groups wherein Z is F or CF.sub.3, t is 1 to 12, and W is --COOR or --CN, wherein R is lower alkyl. Preferably, the functional group in the side chains of the polymer will be present in terminal ##STR2## groups wherein t is 1 to 3.
The term "fluorinated polymer", used herein for carboxylic and for sulfonic polymers, means a polymer in which, after loss of any R group by hydrolysis to ion exchange form, the number of F atoms is at least 90% of the total number of F, H and Cl atoms in the polymer. For chloralkali cells, perfluorinated polymers are preferred, though the R in any COOR group need not be fluorinated because it is lost during hydrolysis.
Polymers containing ##STR3## side chains, in which m is 0, 1, 2, 3 or 4, are disclosed in U.S. Pat. No. 3,852,326.
Polymers containing --(CF.sub.2).sub.p COOR side chains, where p is 1 to 18, are disclosed in U.S. Pat. No. 3,506,635.
Polymers containing ##STR4## side chains, where Z and R have the meaning defined above and m is 0, 1, or 2 (preferably 1) are disclosed in U.S. Pat. No. 4,267,364.
Polymers containing terminal --O(CF.sub.2).sub.v W groups, where W is defined above and v is from 2 to 12, are preferred. They are disclosed in U.S. Pat. Nos. 3,641,104, 4,178,218, 4,116,888, British Patent Nos. 2,053,902, EP No. 41737 and British Patent No. 1,518,387. These groups may be part of ##STR5## side chains, where Y=F, CF.sub.3 or CF.sub.2 Cl. Especially preferred are polymers containing such side chains where v is 2, which are described in U.S. Pat. Nos. 4,138,426 and 4,487,668, and where v is 3, which are described in U.S. Pat. No. 4,065,366. Among these polymers, those with m=1 and Y=CF.sub.3 are most preferred.
The above references describe how to make these polymers.
The sulfonyl polymers with which the present invention is concerned are fluorinated polymers with side chains containing the group ##STR6## wherein R.sub.f is F, Cl, CF.sub.2 Cl or a C.sub.1 to C.sub.10 perfluoroalkyl radical, and X is F or Cl, preferably F. Ordinarily, the side chains will contain --OCF.sub.2 CF.sub.2 CF.sub.2 SO.sub.2 X or --OCF.sub.2 CF.sub.2 SO.sub.2 F groups, preferably the latter. For use in chloralkali membranes, perfluorinated polymers are preferred.
Polymers containing the side chain ##STR7## where k is 0 or 1 and j is 3, 4, or 5, may be used. These are described in British Patent No. 2,053,902.
Polymers containing the side chain --CF.sub.2 CF.sub.2 SO.sub.2 X are described in U.S. Pat. No. 3,718,627.
Preferred polymers contain the side chain ##STR8## where R.sub.f, Y and X are defined above and r is 1, 2, or 3, and are described in U.S. Pat. No. 3,282,875. Especially preferred are copolymers containing the side chain ##STR9##
Polymerization can be carried out by the methods described in the above references. Especially useful is solution polymerization using ClF.sub.2 CCFCl.sub.2 solvent and (CF.sub.3 CF.sub.2 COO).sub.2 initiator. Polymerization can also be carried out by aqueous granular polymerization as in U.S. Pat. No. 2,393,967, or aqueous dispersion polymerization as in U.S. Pat. No. 2,559,752 followed by coagulation as in U.S. Pat. No. 2,593,583.
To make the lowest equivalent weight ion exchange polymers, copolymer in the melt-fabricable (for example, --SO.sub.2 F or --COOCH.sub.3) form may be extracted as in U.S. Pat. No. 4,360,601 and the extracted polymer isolated for use in making the diaphragm. The extract has lower equivalent weight than the starting material.
The copolymers used herein should be of high enough molecular weight to produce films which are self-supporting in both the melt-fabricable precursor form and in the hydrolyzed ion-exchanged form.